Quantitation of hydrophobicity in micellar liquid chromatography

Study of the Relationships between the Structure, Lipophilicity and Biological Activity of Some Thiazolyl-carbonyl-thiosemicarbazides and Thiazolyl-azoles

Lipophilicity, as one of the most important physicochemical parameters of bioactive molecules, was investigated for twenty-two thiazolyl-carbonyl-thiosemicarbazides and thiazolyl-azoles. The determination was carried out by reversed-phase thin-layer chromatography, using a binary isopropanol-water mobile phase. Chromatographically obtained lipophilicity parameters were correlated with calculated log P and log D and with some biological parameters, determined in order to evaluate the anti-inflammatory and antioxidant potential of the investigated compounds, by using principal component analysis (PCA). The PCA grouped the compounds based on the nature of their substituents (X, R and Y), indicating that their nature, electronic effects and molar volumes influence the lipophilicity parameters and their anti-inflammatory and antioxidant effects. Also, the results of the PCA analysis applied on all the experimental and computed parameters show that the best anti-inflammatory and antioxidant compounds were correlated with medium values of the lipophilicity parameters. On the other hand, the knowledge of the grouping patterns of the tested variables allows the reduction of the number of parameters, determined in order to establish the biological activity.

Recent theoretical and practical applications of micellar liquid chromatography (MLC) in pharmaceutical and biomedical analysis

Micellar liquid chromatography (MLC) is an analytical technique belonging to the wide range of reversed-phase liquid chromatographic (RP-LC) separation techniques. MLC with the use of surfactant solutions above its critical micellar concentration (CMC) and the addition of organic modifiers is currently an important analytical tool with still growing theoretical considerations and practical applications in pharmaceutical analysis of drugs and other biologically active compounds. The use of MLC as an alternative, relatively much faster in comparison to conventional chromatographic separation techniques has several advantages, especially as being suitable for screening pharmaceutical analysis. The analytical data received from MLC analysis are considered a useful source of information to predict passive drug absorption, drug transport and other pharmacokinetics and physicochemical measures of pharmaceutical substances. In the review several MLC assays for determination of drugs and other active compounds in biological samples were compared and critically discussed. The presented overview provides information on recent applications and achievements connected with the practical use of MLC. The review covers fields of interest related to theory and mechanism of MLC separation, direct applications of MLC in pharmaceutical analysis, including optimization and efficiency of separation with the use of modification of stationary phase and mobile phase compositions as well as the determination of physicochemical characteristics of drugs by MLC.

Heteroscedasticity of retention factor and adequate modeling in micellar liquid chromatography

The two concepts of micelle formation (pseudo-phase and mass-action) could be the basis of retention models in micellar liquid chromatography (MLC). The separation of 4-hydroxybenzoic acid esters and seven polyaromatic hydrocarbons were performed to study the repeatability of retention factor in MLC. The full two factor experimental design was used for studying the dependence of retention factor variance on mobile phase composition (sodium dodecylsulfate, 1-butanol). The experimentally observed heteroscedasticity and perturbations after linearization were taken into account by using statistical weights obtained on the basis of errors propagation law and the modeling of retention by non-weighted and weighted least squares method was performed. The mechanistical retention models based on pseudo-phase and mass-action concepts of micelle formation were compared by fitting quality and prediction capability and high robustness of bilogarithmic dependence was observed. The significance of retention factor heteroscedasticity for retention hydrophobicity relationships was shown.

Separation methods for estimating octanol-water partition coefficients

Separation methods for the indirect estimation of the octanol-water partition coefficient (logP) are reviewed with an emphasis on high throughput methods with a wide application range. The solvation parameter model is used to identify suitable separation systems for estimating logP in an efficient manner that negates the need for empirical trial and error experiments. With a few exceptions, systems based on reversed-phase chromatography employing chemically bonded phases are shown to be unsuitable for estimating logP for compounds of diverse structure. This is because the fundamental properties responsible for chromatographic retention tend to be different to those responsible for partition between octanol and water, especially the contribution from hydrogen bonding interactions. On the other hand, retention in several micellar and microemulsion electrokinetic chromatography systems is shown to be highly correlated with the octanol-water partition coefficient. These systems are suitable for the rapid, high throughput determination of logP for neutral, weakly acidic, and weakly basic compounds. For compounds with a permanent charge, electrophoretic migration and electrostatic interactions with the stationary phase results in inaccurate estimation of partition coefficients. The experimental determination of solute descriptors offers an alternative approach for estimating logP, and other biopartitioning properties. A distinct advantage of this approach is that once the solute descriptors are known, solute properties can be estimated for any distribution or transport system for which a solvation parameter model has been established.

Quantitative structure—retention and retention—activity relationships of some 1,3-oxazolidine systems by RP-HPTLC and PCA

A quantitative structure-retention and retention-activity relationships investigations were performed on the lipophilicities of some 1,3-oxazolidine systems as estimated by RP-HPTLC retention parameters. The classical R(Mo) values were compared with the factors scores obtained by principal component analysis based also onto the TLC retention data. The lipophilicities (R(Mo) and factor scores) were correlated with the theoretical molecular descriptors of 1,3-oxazolidine derivatives providing by the ALCHEMY 2000 software package. The reliability of the factor scores values as lipophilic indices are shown by their significant correlation with the classical R(Mo) values and other molecular descriptors. In addition, the "lipophilicity chart" described by the first two components, and/or the "lipophilicity space" described by the first three components have the effect of separating compounds from each other most effectively from the congeneric (similarity) aspect point of view. Finally, these findings support the idea that the chromatographic process of the investigated compounds in this paper and consequently their partitioning over a bio-membrane are controlled mainly by lipophilicity.

Evaluation of lipophilicity of some benzimidazole and benztriazole derivatives by RP HPTLC and PCA

The lipophilic character of some benzimidazole and benztriazole derivatives was studied. The classical R(Mo) values were compared with the factors scores obtained by principal component analysis (PCA) based also onto the TLC retention data. The very high correlation between the R(Mo) values and slopes (specific hydrophobic surface area) indicated as usually that this group of compounds could be considered as a homologous series independently of their structural heterogeneity. It is emphasized once again that this procedure can not be a rational and objective way for congeneric studies because always there is a high correlation between slope and intercept. The reliability of the factor scores values as lipophilic indices are shown by their high correlation with the classical R(Mo) values. In addition, the 'lipophilicity chart' described by the first two components has the effect of separating compounds from each other most effectively from the congeneric aspect point of view. The most lipophilic compounds appeared to be the benzimidazole derivatives.

Micellar liquid chromatography: suitable technique for screening analysis

The screening capability of micellar liquid chromatography (MLC) is discussed using the reported chromatographic data of several sets of compounds (amino acids, beta-blockers, diuretics, phenethylamines, phenols, polynuclear aromatic hydrocarbons, steroids and sulfonamides) and new results (sulfonamides and steroids). The chromatographic data are treated with an interpretive optimisation resolution procedure to obtain the best separation conditions. Usually, the pH and the concentration of surfactant (sodium dodecyl sulfate, SDS, or cetyltrimethylammonium bromide) for the optimal mobile phase were 2.5-3 and < 0.12 M, respectively. The nature and concentration of organic solvent depended on the polarity of the eluted compounds: a low volume fraction of propanol (approximately 1%, v/v) was useful to separate the amino acids, with log P(o/w) < -1 (where P(o/w) is the octanol-water partition coefficient). A greater concentration of this solvent (approximately 5-7%) was needed for compounds in the range -1 < log P(o/w) < 2, as with the studied diuretics and sulfonamides, and a high concentration of propanol (approximately 15%) or a low concentration of butanol (< 10%) had to be used for less polar compounds with 1 < log P(o/w) < 3, such as the beta-blockers. Pentanol (< 6%) was more suitable for the even less polar compounds with log P(o/w) > 3, such as the steroids. For basic drugs such as the phenethylamines (0 < log P(o/w) < 1.7), eluted with a micellar eluent of anionic SDS, propanol was too weak. A study is also shown for mixtures of sulfonamides (log P(o/w) = -1.2 to 1.7) and steroids (log P(o/w) = 3.0-8.1) eluted from conventional C18 columns with SDS mobile phases containing acetonitrile and 1-pentanol, respectively, which are compared with classical acetonitrile-water and methanol-water mixtures. The results complement a previous study on beta-blockers (log P(o/w) = -0.03 to 2.8) and reveal that MLC is a very competitive technique for the screening of compounds against conventional RPLC, due to its peculiar behaviour with regard to the selectivity and elution strength. The concentration of organic solvent needed to obtain sufficiently low retention times (even for highly hydrophobic steroids with log P(o/w) = 7-8) is also appreciably smaller for MLC, which reduces the environmental impact of the mobile phases.

Evaluation of the lipophilicity of bile acids and their derivatives by thin-layer chromatography and principal component analysis

The lipophilic character of bile acids and their glyco- and tauro-conjugates was studied. The classical R(Mo) values were measured by means of reversed thin-layer chromatography using a mixture of methanol-water as the solvent system and compared with the factors scores obtained by principal component analysis based also onto the TLC-retention data. The reliability of the factor scores values as lipophilic indices are shown by their high correlation with the classical R(Mo) values. In addition, a better correlation was observed between scores corresponding to the first principal components and the partition coefficients (log P) of bile acids. Finally, the "lipophilicity chart" described by the first two components has the effect of separating compounds from each other most effectively from the congeneric aspect point of view.

Quantitative structure-retention and retention-activity relationships of beta-blocking agents by micellar liquid chromatography

Sixteen beta-blocking agents (acebutolol, alprenolol, atenolol, bisoprolol, carteolol, celiprolol, esmolol, labetalol, metoprolol, nadolol, oxprenolol, pindolol, practolol, propranolol, sotalol and timolol) showing a large range of hydrophobicity (octanol-water partition coefficients, log P between -0.026 and 2.81) were subjected to micellar liquid chromatography with sodium dodecyl sulfate as micelle forming agent, and n-propanol as organic modifier. The correlation between log P and the retention factor extrapolated to a mobile phase free of micelles and organic modifier was investigated. The use of an interpolated retention factor or the retention factor for specific individual experimental mobile phases was however advantageous since the retention factors of all beta-blocking agents were measurable in the selected mobile phases. Good correlations with log P and with in vitro biological parameters (cellular permeability coefficients in Caco-2 monolayers and apparent permeability coefficients in rat intestinal segments) were found.

Exploratory chemometric analysis of the classification of pharmaceutical substances based on chromatographic data

A chemometric study has been conducted on a published data set consisting of the retention times of 83 substances, from five pharmacological families, on eight HPLC systems. Principal component analysis, clustering and sequential projection pursuit were applied. In this way it was investigated to what extent the combination of chromatography and chemometrics allows one to make conclusions about pharmacological activities of (candidate) drugs and what the contribution is of the different HPLC systems considered.